The present invention relates to a method and apparatus to individually detect the cathode to helix current of multiple TWTs sharing a common cathode to helix voltage supply.
Travelling wave tubes (TWTs) are used as amplifiers and modulator in a number of high-frequency communications applications, such as satellites. Typically, electrical power is supplied to travelling wave tubes by a device known as a High Voltage Electronic Power Converter (HV EPC). The HV EPCs for some satellite and other communication systems have been designed so that a single power supply is used to provide DC voltages and currents to two or more TWTs. The use of a single HV EPC to supply multiple tubes provides mass and volume reductions compared to having TWTs powered by individual HV EPCs.
To retain this mass and volume reduction, it is typically required that the TWTs share a common cathode to helix potential and a common collector to cathode potential. The sharing of the cathode to helix potential also provides the advantage of minimizing phase differentials between the RF signal outputs of the TWTs, which allows for the combining of the output RF power without deleterious effects on the RF signals amplified by the TWT.
There are several disadvantages to sharing a common cathode to helix voltage, which arise from the fact that any helix current return is also shared by the multiple TWT loads. For example, if the helix current of a tube exceeds it limit, which may occur due to electron beam intercept with the helix, there is no way to tell which of the tubes has exceeded the limit. A more severe problem can occur due to the fact that since the helix current return is shared by the multiple TWT loads, the fault protection limit for the helix current cannot be set at a level that is sufficiently low to adequately protect any one helix structure from destruction due to excessive current.
A need arises for a technique that provides the capability to determine which tube has exceeded the limit and to adequately protect each helix structure from destruction due to excessive current.
The present invention is a method and apparatus to individually detect the cathode to helix current of multiple TWTs sharing a common cathode to helix voltage supplied by a single HV EPC. The present invention provides the capability to determine which tube has exceeded the limit and to adequately protect each helix structure from destruction due to excessive current.
In one embodiment of the present invention, an apparatus for detecting travelling wave tube helix current comprises a capacitor having an input electrically connected to a radio frequency signal, an inductor having an input electrically connected to an output of the capacitor and to an input to a helix of a travelling wave tube, the helix of the traveling wave tube electrically connected to a chassis of the traveling wave tube by an impedance, a resistor having an input electrically connected to an output of the inductor and an output electrically connected to the chassis of the travelling wave tube; and wherein a cathode to helix current of the travelling wave tube may be sensed across the resistor.
The cathode to helix current of the travelling wave tube may be sensed by an analog telemetry voltage generated across the resistor. A value of the resistor is calibrated so that a desired proportion of the cathode to helix current of the travelling wave tube is routed through the resistor.
In one embodiment of the present invention, a method of detecting travelling wave tube helix current comprises the steps of routing a first portion of a cathode to helix current in a helix of a traveling wave tube through a lossy attenuator electrically connected to the helix, routing a second portion of the cathode to helix current in the helix of the traveling wave tube through a resistor electrically connected in parallel to the lossy attenuator, and sensing the cathode to helix current of the travelling wave tube across the resistor.
The sensing step may comprise the step of sensing the cathode to helix current of the travelling wave tube using an analog telemetry voltage generated across the resistor. The method may further comprise the step of calibrating a value of the resistor so that a desired proportion of the cathode to helix current of the travelling wave tube is routed through the resistor.
In one embodiment of the present invention, an apparatus for individually detecting travelling wave tube helix current of multiple travelling wave tubes sharing a common cathode to helix voltage supply comprises a circuit for each travelling wave tube, comprising a capacitor having an input electrically connected to a radio frequency signal, an inductor having an input electrically connected to an output of the capacitor and to an input to a helix of the travelling wave tube, the helix of the traveling wave tube electrically connected to a chassis of the traveling wave tube by an impedance, a resistor having an input electrically connected to an output of the inductor and an output electrically connected to the chassis of the travelling wave tube, and wherein a cathode to helix current of the travelling wave tube may be sensed across the resistor.
The cathode to helix current of the travelling wave tube may be sensed by an analog telemetry voltage generated across the resistor. A value of the resistor may be calibrated so that a desired proportion of the cathode to helix current of the travelling wave tube is routed through the resistor.
In one embodiment of the present invention, a method of individually detecting travelling wave tube helix current of multiple travelling wave tubes sharing a common cathode to helix voltage supply comprises the steps of for each travelling wave tube, routing a first portion of a cathode to helix current in a helix of a traveling wave tube through a lossy attenuator electrically connected to the helix, routing a second portion of the cathode to helix current in the helix of the traveling wave tube through a resistor electrically connected in parallel to the lossy attenuator, and sensing the cathode to helix current of the travelling wave tube across the resistor.
The sensing step may comprise the step of sensing the cathode to helix current of the travelling wave tube using an analog telemetry voltage generated across the resistor. The method may further comprise the step of calibrating a value of the resistor so that a desired proportion of the cathode to helix current of the travelling wave tube is routed through the resistor.